Capacitors typically are manufactured by compressing powders, e.g. tantalum, to form a pellet, sintering the pellet in a furnace to form a porous body, and then subjecting the body to anodization in a suitable electrolyte to form a continuous dielectric oxide film on the sintered body.
Development of tantalum powders suitable for capacitors has resulted from efforts by both capacitor producers and powder processors to delineate the characteristics required of tantalum powder in order for it to best serve in the production of quality capacitors. Such characteristics include surface area, purity, shrinkage, green strength, and flowability.
For tantalum capacitors, the oxygen concentration in the tantalum pellets is critical. For example, when the total oxygen content of porous tantalum pellets is above 3000 ppm (parts per million), capacitors made from such pellets may have unsatisfactory life characteristics. Unfortunately, the tantalum powders used to produce these pellets have a great affinity for oxygen, and thus the processing steps which involve heating and subsequent exposure to air inevitably result in an increased concentration of oxygen. In the production of capacitor grade tantalum powder, electronic grade tantalum powder is normally heated under vacuum to cause agglomeration of the powder while avoiding oxidation of the tantalum. Following this heat treatment, however, the tantalum powder usually picks up a considerable amount of additional oxygen because the initial surface layer of oxide goes into solution in the metal during the heating and a new surface layer forms upon subsequent exposure to air thereby adding to the total oxygen content of the powder. During the later processing of these powders into anodes for capacitors, the dissolved oxygen may recrystallize as a surface oxide and contribute to voltage breakdown or high leakage current of the capacitor by shorting through the dielectric layer of amorphous oxide. Accordingly, the electrical properties of tantalum capacitors would be markedly improved if the oxygen content could be controlled, i.e., decreased, maintained about constant or increased within acceptable limits.
One technique which has been employed to deoxidize tantalum powder has been through the mixing of alkaline earth metals, aluminum, yttrium, carbon, and tantalum carbide with the tantalum powder. However, there are certain disadvantages to this technique. The alkaline earth metals, aluminum, and yttrium form refractory oxides which must be removed, e.g., by acid leaching, before the material is suitable for capacitors. With respect to carbon, the amount of carbon must be carefully controlled since residual carbon is also deleterious to capacitors even at levels as low as 50 ppm. Still, other methods which have been proposed involve using a thiocyanate treatment or using a hydrocarbon or reducing atmosphere during some of the tantalum processing stages in order to prevent oxidation and thus keep the oxygen content low.
Another process scheme proposed in U.S. Pat. No. 4,722,756 (Hard) for the control of the oxygen content of tantalum and columbium materials provides for heating the material in an atmosphere containing hydrogen gas in the presence of a metal more oxygen active than tantalum or columbium, e.g. titanium or zirconium. However, a disadvantage of the Hard process is that the metals utilized in controlling the oxygen content may contaminate the tantalum or columbium material.